1. Field of the Invention
The present invention relates to a photosensitive paste, and more particularly to improvements in the monomer component included in photosensitive paste.
2. Description of Related Art
Paste is widely used to form conductive pattern such as electrode or resistance pattern. Paste includes functional components which impart functions to formed pattern and other components which impart excellent properties to paste itself. For instance, conductive paste for electrode includes conductive powder, glass frit, organic binder, organic solvent, and additives such as stabilizer. Conductive powder imparts conductivity to formed electrode. Glass frit works as binder to keep conductive powder in a sintered electrode. Organic binder works as binder to keep each component dispersed in paste.
With development of electric material, the demand for fine pattern has been increasing. Photosensitive paste was developed to meet this demand. Photosensitive paste includes monomer and photopolymerization initiator as component of paste, the polymerization proceeds in an area where light is irradiated (negative type photosensitive paste). After the following development process, the area remains as pattern. As compared with screen printing, finer pattern can be formed by use of photosensitive paste.
However, undercut has been emerging as a serious problem in pattern forming by photosensitive paste. Undercut means the width of pattern gets narrow at the bottom part during development in a way that the pattern is eroded inwardly. Undercut is commonly seen in a pattern formed from photosensitive paste with inorganic pigments, because irradiated light is strongly absorbed or reflected in paste layer. When the width of pattern is large, effect of undercut is relatively small and negligible. However, when the width of pattern is small, undercut comes to have impact on the property of pattern. For instance, provided that the amount of undercut is 10 micrometer, pattern width of 100 micrometer is reduced to 90 micrometer by development. Under the same premise, pattern width of 30 micrometer is reduced to 20 micrometer. If the pattern is conductive layer, such a huge change of pattern width means less contact area between the conductive pattern and the adjacent substrate, resulting in increase of resistance. In addition, the reduced contact area between the pattern and the substrate caused by undercut induces less adhesiveness, which leads to defect of electric product.
By the way, the front panel of plasma display panel (PDP) typically has bus electrode with two-layer structure, where white electrode and black electrode are layered in order to improve contrast of panel. Typical structure is, from the side of viewer of PDP, front glass/transparent electrode/black electrode/while electrode. If the undercut of black electrode is large in this structure, a part of while electrode can be seen by viewer, resulting in debasement of contrast.
As conventional monomer used in photosensitive paste, ethylenically unsaturated monomer such as hexanediol triacrylate and ethylene glycol diacrylate, is widely used (cf. U.S. Pat. Nos. 6,790,596, 6,749,994, 6,132,937, 6,197,480). There is a need to provide photosensitive paste with less undercut during development.